Place an object on each face of a polyhedron
Let's say you have an octahedron and a gear and you want to have a gear on each of the faces of the octahedron.



Select the gear, then shift select the octahedron (so that both are selected but the octahedron is the active object)



Then press CtrlP and select parent to vertex.



Set the duplication to faces.



The advantege of this is that both objects remain editable, and chhanges on the duplicated object will be applied to each one of the instances. Once you are happy with the results you can Make Duplicates Real by pressing Ctrl+Shift+A

EDIT:

Both objects must be at a common origin.